1. Field of the Invention
The present invention relates to platinum-substitute electrocatalysts for fuel cells and to pyrochlore-type oxides that can be used as catalysts thereof.
2. Description of the Related Art
A fuel cell is a device that generates electricity through an electrochemical reaction between hydrogen (fuel) and oxygen. The product of this reaction is water in principle and thus has a small effect on the environment. Thus, fuel cells are expected for applications in a co-generation system for houses and have been developed.
As a catalytic component used as an electrocatalyst for polymer electrolyte fuel cells, a platinum (Pt) or Pt alloy is generally used (Patent document No. 1). However, a Pt is a metal that is very expensive and rare from the aspect of resources, and it is thus concerned that a Pt may become subject to restrictions in terms of cost and depletion of the resources in the future when fuel cells become widely used in the full scale. A problem related to the practical use of a Pt catalyst is that a cathodic overvoltage is large when a Pt is used as a catalyst for cathode and that a decrease in electricity generation efficiency due to the cathodic overvoltage is a limiting factor for an energy reduction effect. It is also a problem from a durability point of view of a Pt catalyst that Pt atoms on a carrier leak in an electrolytic substance in an electrode layer due to, for example, the long-term use or frequent start-stop of the Pt catalyst, resulting in a decrease in a Pt specific surface area associated with an increase in a Pt particle size such that performance degradation is caused. Further, so-called Pt band formation where leaked platinum atoms deposit in an electrolyte membrane is also recognized as a durability-related problem.
In order to overcome these problems, the development of high-performance (less cathodic overvoltage) and high-durability (low solubility into acid electrolytes) non-Pt electrocatalysts is required.
It has already been reported that pyrochlore-type oxide catalysts, represented by PbRu mixed oxide system and PbIr mixed oxide system, exhibit high activity towards an oxygen reduction reaction (ORR) in fuel cell cathodes as non-Pt electrocatalysts. (Non-patent documents No. 1 and No. 2) In general, the following conditions are required for electrocatalysts for fuel cells to achieve high ORR performance: a high activity for each catalyst specific surface area, a large catalyst specific surface area, and, further, a high catalyst conductivity in the case of an oxide catalyst. However, electron transfer in an electrode often serves as reaction control for an ORR in a pyrochlore-type oxide catalyst. This is because particles easily aggregate together at the time of catalyst preparation such that often times only a pyrochlore-type oxide catalyst having a small specific surface area is obtained and is also because the conductivity of the oxide itself is low.
The present inventors have found a technique of synthesizing pyrochlore-type oxide catalysts by a precipitate formation reaction at room temperature so as to prevent such particle aggregation at the time of conventional pyrochlore-type oxide catalyst preparation. Further, the present inventors have been studied a method of preparing a catalyst that can achieve a high ORR activity in an actual membrane electrode assembly (MEA), as a result, by forming precipitates in a state where conducting substances such as carbon powder is suspended in advance at the time of synthesizing the catalyst for a purpose of maintaining conductivity in an electrode catalyst layer so as to reduce the resistance of electron transfer between the catalyst that is obtained and an electrode layer bulk (JP Application No. 2009-022876).
[Patent document No. 1] JP 05-36418
[Non-patent document No. 1] Y. Shimizu et. al., “ITE Letters on Batteries, New Technologies & Medicine,” Vol. 4, 2003, p. 582
[Non-Patent document No. 2] D. Saeki et. al., “The 49th Battery Symposium in Japan,” proceeding speech 3A21, 2008, p. 89-92
The above-stated preparation method is a process where a precipitate formation reaction is performed at a room temperature and that does not require calcination or the like at high temperature afterward. Therefore, the preparation method is more advantageous than a method, where synthesis of catalysts is performed at higher temperature, in that particle aggregation caused at high temperature can be prevented. (Patent document No. 1) On the other hand, however, the crystallinity of a catalyst that is obtained is low as a pyrochlore-type oxide, and there is room for improvement for the preparation method in terms of the resistance to dissolving of the catalyst and in terms of maintaining an ORR activity.